1. Field of the Invention
The present invention relates to a thermoelectric conversion material for performing energy conversion between electric energy and thermal energy, using thermoelectric effect, and a thermoelectric conversion element using the material. The present invention also relates to an electronic apparatus using thermoelectric generation induced by the material and a cooling device using thermoelectric cooling induced by the material.
2. Description of the Related Art
Thermoelectric generation is a technology for directly converting thermal energy into electric energy by Seebeck effect, a phenomenon in which a temperature difference in opposite ends of a substance causes thermal electromotive force proportional to the temperature difference. This electric energy can be used as electric power when a load is connected thereto and a closed circuit is constituted. This technology has found practical applications in power sources for remote areas, for space, for military use, and the like.
Thermoelectric cooling is a technology for causing heat absorption by Peltier effect, a phenomenon in which application of an electric current through a circuit made of different substances connected to each other causes heat absorption in one junction and heat generation in the other junction. This effect is thought to be derived from the characteristic that the ratio between the electric current carried by electrons and a heat current carried thereby differs between the substances when two kinds of substances each of which is different in carrier polarity, a p-type semiconductor and an n-type semiconductor, for example, are thermally connected in parallel and electrically connected in series, and the electric current is applied therebetween. The thermoelectric cooling technology has found practical application in local cooling devices such as for cooling electronic devices in a space station, and wine coolers.
To date, desired is a thermoelectric conversion material exhibiting a good thermoelectric conversion characteristic (thermoelectric performance) in a wide range of temperatures from room temperature to high temperature. Various materials, most of which are semiconductors, are under consideration.
The thermoelectric performance is generally evaluated by a figure of merit Z, or a figure of merit ZT that is made dimensionless by multiplying Z by absolute temperature T. ZT can be expressed as ZT=S2/ρκ, where S is a Seebeck coefficient, ρ is electric resistivity, and κ is thermal conductivity. That is, in order to allow the thermoelectric conversion material to exhibit excellent thermoelectric performance, large thermal electromotive force, small thermal conductivity, and small electric resistivity are desired. In the conventional thermoelectric conversion materials, however, it can not be said that sufficient ZT is obtained. This is because S, ρ, and κ are basically functions of carrier density, and thus, difficult to vary independently, which gives rises to difficulties in finding the appropriate solution.
The thermoelectric conversion materials developed until today include a Bi2Te3-based semiconductor. With this material, thermoelectric performance on a practical level at room temperature can be obtained. In addition, there have been developments toward the practical application on materials having a complicated structure such as a Skutterudite compound, and a clathrate compound.
JP 8(1996)-186294 A (reference 1) discloses a thermoelectric conversion material expressed by a formula of Co1-xMxSb3 (x is 0.001 to 0.2 in the reference 1) in which one portion of Co, which is an element constituting a CoSb3 compound having a Skutterudite structure, is substituted with at least one element M selected from Pd, Rh, and Ru. The thermoelectric conversion material disclosed in the reference 1, however, has a problem in that in a high-temperature range, the thermoelectric performance of the material deteriorates due to oxidization.
JP 9(1997)-321346 A (reference 2), JP 2003-218411 A (reference 3), and WO03/085747 (reference 4) disclose a thermoelectric conversion material referred to as a so-called “AMO2-type oxide” (in the references 2 to 4, A is an alkali metal or an alkaline-earth metal, and M is Co). These materials do not easily suffer from damage or oxidization even under a high-temperature environment, and exhibit excellent thermoelectric performance. The thermoelectric conversion material disclosed in the references 2 to 4 are materials having a so-called “layered bronze structure” described later. It is well known that an AMO2-type crystal, typical of the layered bronze structure, has metallic properties, that is, the property that the electric resistivity increases with increasing temperature.
WO2004/095594 (reference 5) discloses a thermoelectric conversion material including a Half-Heusler alloy expressed by a formula of QR(L1-pZp) (in the reference 5, Q is a group 5 element, R is at least one element selected from Co, Rh and Ir, and p is 0 (zero) or more, and less than 0.5).
JP 2005-64407 A (reference 6) discloses a thermoelectric conversion material expressed by a formula of SrxRh2Oy (in the reference 6, x is 0.7 to 1.0, and y is 4.0 or more), and describes that the electric resistivity of the material exhibits the above-described metallic property (see paragraph number [0020], for example).
The thermoelectric performance of these thermoelectric conversion materials, however, is not yet satisfactory, and is lower than that of the Bi2Te3-based semiconductor that has entered a practical stage. This calls for further improvement of the thermoelectric performance. In addition, while it is expected that the thermoelectric generation in a temperature range higher than conventional produces a greater electric energy, the thermoelectric conversion material exhibiting metallic properties increases the electric resistivity of the material with increasing temperature, thereby resulting in a larger loss.